The present invention pertains to a process for the preparation of a dark-coloured, wave-length selective oxide film on aluminium.
Providing a dark-coloured, wave-length selective oxide film on aluminium results in products which are suitable for use, inter alia, as thermal solar panels or, due to their characteristic appearance, for decorative purposes.
In the ensuing discussion the term "spectral selectivity" will first of all be described in connection therewith that aluminium, obtained by the process according to the present invention, will be essentially perceived to the eye as having a black colour. Thereafter processes for the black-colouring of aluminium will be described, in order to show that the invention is founded on other principles than those previously known. Finally, the results from optical measurements which have been carried out, as well as some illustrative Examples, will be presented.
As is known, an absolutely black surface will absorb most radiation, but it will, according to Kirchhoff's equation, also have the highest loss of heat through radiation.
Collectors for solar heat should have a high absorption coefficient for the frequencies which the main portion of the energy of the sun rays have, at the same time as the radiation of heat should be as low as possible. The heat radiation has a much lower frequency than the main portion of the solar radiation. Thus, it is desirable that the absorption coefficient vary with the frequency of the radiation, so that the absorption coefficient will be high for high frequency and low for low frequency, i.e., a spectral selectivity which is suitable for thermal solar panels.
In addition to the classic division into black, grey, white, reflecting and bright surfaces, there exist also surfaces having a spectrally selective film. The appearance of a surface having a spectrally selective film will vary with the wave-length for the change from low to high reflection, and for solar panels it is desired that the change is to occur at a higher wave-length than is perceived by the eye. A surface of a spectrally selective film for solar panels will usually be conceived by the eye as nearly black.
The reduced radiation of long-wave infrared radiation is important for a solar panel, as it thereby is possible to obtain a higher temperature than for an absolutely black surface. Whereas an absolutely black surface will attain equilibrium between incoming and emitted radiation at about 80.degree. C. at about 60.degree. latitude, it is not unrealistic to expect temperatures of over 200.degree. C. for equilibrium for radiation equilibrium for aluminium treated with the process according to the present invention. The increased temperature is of decisive importance for the possibilities of economical storage of solar heat.
The "classic" example of a surface having a spectrally selective absorbing film is so-called "black chromium" (chromium black) reported some years ago by G. E. McDonald (Sol. Energy 17, 119 (1975)). The correct explanation of the phenomenon was found by Fan et Spura (Appl. Phys. Lett. 30, 511 (1977)) who stated that these layers consisted of polycrystalline Cr.sub.2 O.sub.3 admixed with extremely fine-grained chromium metal.
As aluminium treated by the process according to the present invention is given a nearly black appearance, the main features of known processes for black-colouring of aluminium will be described. A series of mutually related processes is based on the precipitation of black salts on the surface and another series of processes is to apply a porous oxide film on aluminium by anodization and fill dyestuff into the pores. To illustrate the first group of processes, it is to begin with referred to an example where also the thermal characteristics are noted.
According to Dutch Patent No. 21 066 76 copper-containing aluminium alloys can be black-coloured by means of a two-stage chemical process wherein the first bath consists of water to which NiCl.sub.2, NH.sub.4 Cl and KCNS have been added, and the next bath consists of water to which HNO.sub.3 has been added. This process aims at giving a material having a high emission of thermal long-wave radiation, which is important for cooling surfaces, but undesired for solar heat collectors, for which it as previously mentioned is desired to have low emission for radiation having long wave-lengths.
Industrial anodizing of aluminium, for instance in sulphuric acid, chromic acid, oxalic acid or mixtures thereof, gives oxide films having thicknesses which usually are from 5 to 30 micrometers thick. Only an inner layer of up to 700 Angstrom is porefree, whereas the essential portions of the layer has tunnel-shaped pores being open to the surface. These films can be inked up and blackened by filling the pores with organic or inorganic dyestuff after or under the very anodizing. For instance, French Patent No. 22 149.10 describes a colouring process based on an anodic treatment in a bath consisting of water to which CuSO.sub.4, NiSO.sub.4, (NH.sub.4).sub.2 SO.sub.4 and H.sub.3 BO.sub.3 have been added.
Thus, a black compound is used and the result is therefore black also in a physical sense and not in the sense of spectral selectivity.
A further type of anodizing is the preparation of so-called barrier layers. These are impervious and transparent and up to 7000 Angstrom thick.
There exist further hard anodizing processes which give dark and sometimes nearly black oxide films. The more usual process uses a sulphuric acid bath at a temperature of about 0.degree. C., about 70 volts and 5-20 A/dm.sup.2. The dark colour is in the various hard-anodizing processes only achieved with relatively thick films, usually only at about 50 micrometers. It has for comparison been effected a series with hard-anodizing and these showed that super-pure aluminium was never coloured dark. Dark colour requires either contaminated or alloyed aluminium grades. It was shown that certain phases of contamination and alloying elements were transferred to the film in unoxidized form. As far as it is known, no satisfactory explanation has been published regarding the dark colouring which can occur in hard-anodizing. It is possible that the dark colour which is achieved in such hard-anodizing for the formation of black films on aluminium basis also shows some degree of wave-length selective characteristics, but the development work in this field was not continued because of the fact that according to the theories and equations of Maxwell and Garnett, the films must be too thick to have optimum characteristics as solar collectors. Anyway, hard-anodizing is a rather expensive process.